crash report: launching non-existent Julia binary

[StefanKarpinski]

Similar crash to a few others but with a custom Julia build as the version being launched:

name = "Juliaup launcher"
operating_system = "macOS 26.3.1 [arm64]"
crate_version = "1.19.10"
explanation = """
Panic occurred in file 'src/bin/julialauncher.rs' at line 700
"""
cause = 'Could not launch Julia. Verify that there is a valid Julia binary at "/Users/stefan/dev/julia/julia".'
method = "Panic"
backtrace = """
   0:        0x1003247cc - backtrace::capture::Backtrace::new::hd048ba9fa1c9a137
   1:        0x1002ed270 - human_panic::setup_panic::{{closure}}::h879873a9c9c7c287
   2:        0x1003f9198 - std::panicking::panic_with_hook::h657ee82e431041f8
   3:        0x100414d7c - std::panicking::panic_handler::{{closure}}::hd0ef6f87b4fd1427
   4:        0x100414d38 - std::sys::backtrace::__rust_end_short_backtrace::hbd8bf96e4ee54567
   5:        0x100414d2c - __rustc[16f1505adc47261a]::rust_begin_unwind
   6:        0x100438900 - core::panicking::panic_fmt::ha09fb24313daf262
   7:        0x1002fa06c - julialauncher::main::h3baa637a73d31375
   8:        0x1002fb9a8 - std::sys::backtrace::__rust_begin_short_backtrace::h272eaf2e6b9c33b6
   9:        0x1003031f0 - _main
"""

A bit of extra info is that /Users/stefan/dev/julia/julia does not exist because I'm in the process of rebuilding Julia. The real issue here may be why make test-* is trying to launch julia without building it first, but figured the crash here is also worth reporting.

[davidanthoff]

Essentially we just need a better error message, right?